Idea

Sometimes it's better to have a design show on a black or white shirt in the preview as it's harder to make out on say a red shirt. Having it default to a random/popular color doesn't help much, or it might show a childs size in a color that isn't optimal for listing previews as people might think it's a child only tshirt (who's going to click a kids tshirt to see if they have adult sizes?).